The present invention relates generally to digital signal generation and more specifically to the generation of digital distortion signals for testing teletypewriters.
In older style teletypewriter systems, printed characters are produced in response to a digital signal, representative of alphanumeric characters, driving a mechanical cam in the teletypewriter. Too much distortion in the digital signal has an adverse effect on the quality of the printed output. To insure a quality printed output, a known amount of distortion is added to the digital signal, for example 30% distortion, and sent to the teletypewriter. The cam is adjusted until the printed character starts to deteriorate. The cam position is marked and the cam is rotated in the opposite direction until the printed character starts to deteriorate again. This position is marked. The cam is then positioned midway between the end points where the printed outputs started to deteriorate.
Traditional methods of generating distortion in teletypewriter digital signals have relied heavily on discrete logic circuits and components to modify a known pattern by a fixed percentage of distortion. The Stelma Telecommunications PG-303A Pattern Generator is an example of a product using such a method. In the PG-303A, front panel controls and switches are used to set-up the instrument to produce various types of output signals at selected baud rates that are to be transmitted to a system under test. The instrument can be set-up to run in synchronous mode where a character pattern is continuously transmitted or in start-stop mode where a release button must be depressed on the front panel to allow a single bit pattern to be transmitted. In addition, either distorted patterns or "true-time" patterns, ones without distortion, can be generated.
To generate and transmit a selected character bit pattern output from the PG-303A, the operator must first manually select the bit pattern to be sent by depressing selected switches of the select character switch matrix on the front panel. The selected bit pattern is loaded into a signal pattern matrix. The bit pattern in the switch pattern matrix is then loaded in parallel format into an eleven stage data register of a character distributor and timing control circuit. The character distributor and timing control circuit converts the parallel data into serial data and outputs it to a data output circuit to be sent to the system under test.
A time base circuit is provided in the instrument to produce clock pulses that are one hundred times the baud rate selected by the operator. The output of the time base circuit is coupled to decade counters in a distortion generator circuit which furnishes time-controlled shift pulses that are used to clock character bits out of the data register of the character distribution and timing control circuit. The decade counter is coupled to a BCD-to-decimal decoder which in combination with the decade counter counts uniformly from 1 to 100, then recycles and repeats the count. The "true-time" or zero distortion transitions coincide with the zero count condition of the counter. The BCD-to-decimal counter is coupled to a "true-time" shift pulse gating network and a early-late shift pulse gating network consisting of front panel switches, logic gates, and flip-flops. The early-late shift pulse gating network develops an early or late shift pulse in response to the front panel switch settings and the units and tens output of the BCD-to decimal counter. The early-late shift pulse clocks the shift register in the character distribution and timing control circuit to output a bit having a percentage of distortion equal to the front panel control settings to the output control circuit.
The early-late shift pulse gating network and the "true-time" shift pulse gating network are controlled by a distortion control flip-flop in the distortion generator circuit that enables either one or the other of the networks. The "true-time" pulses are derived from the zero outputs of the BCD-to-decimal decoder. The selection of the "true-time" versus distortion is determined by sampling the state of the next to last stage of the data register for a mark or a space. Sampling of this stage is necessary to provide the information needed to select a distorted or non-distorted shift pulse.
Character bit patterns are loaded into the data register when an "all space" condition exists in each stage of the register. This occurs in the start-stop mode when the last mark of the start-stop character has been shifted out of the register. In synchronous operation where a bit pattern is continuously loaded into the data register, an "indicator mark" must be added at the end of each group of data bits to signify that the last bit of the synchronous character is in the last stage of the register and is thus being transmitted.
More recently, microprocessor controlled test equipment has been developed for the teletypewriter industry. The Tektronix 834 Communications Network Analyzer with an associated 834R03A ROM Pack is an example of such a product. The 834R03A ROM Pack contains software that configures the 834 for analysis of serially transmitted digital data from a teletypewriter. Under program control, the 834 receives the serially transmitted data and samples it at a substantially higher rate than the baud rate of the system under test, for example one hundred times the baud rate. The sampled data is stored in RAM and delivered to the microprocessor where the number of zeros and ones are counted. The difference between the actual number of zero or one bits and the higher sample rate, in this case 100, represents the amount of distortion in percent. Bias distortion, isochronous distortion, and start/stop distortion can be analyzed in this manner.
What is needed is a method for generating and transmitting various types of distortion on a digital signal in a microprocessor based instrument without adding a large number of discrete logic circuits to the existing hardware in the instrument. In addition, the method should not require the sampling of the contents of a data register containing the character pattern to be transmitted and feeding back the contents to the distortion generating circuits to establish a distorted on non-distorted output as is done in the prior art. Further, the new method should be capable of implementing a variety of test set-ups without having to insert an "indicator mark" at the end of each group of data bits for controlling the operation of the instrument.